Assessment of Rhizobium strains for competitive ability is well known to the art (e.g. Brewin NJ et al. (1983) J. Gen. Microbiol. 129:2973-2977). Effective and ineffective Rhizobium strains have been assessed for symbiotic and competitive abilities by measures such as nodulation, foliage dry weight, foliage nitrogen content, strain recovery from nodules, and the like (Diatloff A and Brockwell J (1974) Austral. J. Exp. Agric. Animal Husb. 16:514-521; Berestetskii OA et al. (1983) Mikrobiologiya (Moscow) 52:651-657). Competition bioassays using a Rhizobium reference strain capable of producing nodules that were visually distinguishable from those produced by normal stains have been used (Eaglesham ARJ et al. (1982) Appl. Environ. Microbiol. 44:611-618). All of these measures are relatively labor-intensive to assess and are not practical for mass screening of isolates or presumptive mutants. Mixed inoculations of a toxin-producing, chlorosis-inducing strain and various normal strains have been used to assess competitiveness (Means UM et al. (1961) Soil Sci. Soc. Proc. 25:105-108). A "blocking" Istrain has also been used to assess competitiveness (Dowling DN et al. (1985 ) in Nitrogen Fixation Research Progress, eds.: Evans HJ et al., p. 141). Those methods are not generally useful, being limited to Rhizobium species having toxin-producing strains or "blocking" strains.